Sequences encoding the production of a cytotoxin (VT) active on Vero cells were cloned in Escherichia coli K12 from a VT-determining phage that originated in E. coli strain H19 of serotype O26.H11. Subcloning resulted in the identification of a 2.5 kb fragment that still coded for VT production. Mutagenesis with transposon Tn1000 was used to map VT sequences and a 0.75 kb probe was developed. In colony hybridization tests with strains isolated from patients with haemolytic uraemic syndrome or diarrhoea, this probe derived from the H19 VT genes detected only some of the VT+ strains belonging to serogroup 0157. A VT+ strain, E32511, serotype 0157.H-, which was negative in colony hybridization was the source of another VT-determining phage from which VT sequences were cloned. Southern hybridization of the VT genes from E32511 with the H19 probe was negative under stringent conditions but there was weak homology under conditions of low stringency. These results indicate that there are differences in the VT genes of pathogenic E. coli.